1. Field of the Invention
The present invention relates to a projection exposure apparatus for projecting an image of a pattern formed on a mask through a projection optical system onto a photosensitive substrate, and more particularly to a projection exposure apparatus which has a function to measure imaging characteristics of the projection optical system.
2. Related Background Art
For projection exposure apparatus used in the lithography process for fabricating semiconductor devices or liquid crystal displays, etc., it remains necessary to measure imaging characteristics of the projection optical system, such as the best focus position, distortion, and projection magnification. For example, the best focus position is used as a target value for positioning the photosensitive substrate such as a wafer or a glass plate to be exposed in the direction of the optical axis of the projection optical system (or in the Z-direction). Since distortion could be a cause of registration error when different projection exposure apparatus are used for exposures in different layers on the photosensitive substrate, such adjustment is necessary as to keep distortion below a predetermined permissible value. Similarly, the projection magnification needs to be controlled within a predetermined range.
One conventional method for measuring imaging characteristics is a method for performing test print. For example, in order to measure the best focus position, a test print operation is carried out as follows: an actual photosensitive substrate is placed on a wafer stage, and the apparatus repeats a step of shifting the photosensitive substrate in the Z-direction vertically to effect exposure of a pattern on a reticle and thereafter horizontally shifting the photosensitive substrate, thereby performing sequential test print while changing Z-directional positions in predetermined steps. Then the best focus position can be determined at a Z-directional position where a photoresist pattern obtained by developing the photosensitive substrate is clearest. As for the distortion and projection magnification, exposure is carried out on the photosensitive substrate, using a test reticle in which patterns for measurement are formed in predetermined arrangement, and they can be obtained by measuring deviation of the arrangement of patterns obtained after development.
Another example of the conventional measuring methods is the so-called stage slit method in which a reference member with a slit formed thereon is mounted on the wafer stage and light passing through the slit is detected by a photoelectric sensor disposed on the bottom side of the slit. In this stage slit method, for example, an image of a pattern of a test reticle is projected through the projection optical system to the wafer stage side, stage scanning is repeated many times in every direction of the image, and light passing through the slit is detected by the photoelectric sensor, thereby measuring distortion etc. without performing actual exposure.
The test print method including actual exposure of photosensitive substrate among the conventional measuring methods of imaging characteristics of projection optical system had a disadvantage of lack of quickness because of inclusion of the development step.
The stage slit method had a disadvantage that the measuring time was still long, because stage scanning needed to be repeated many times in every direction of the image, that is, because information of image could not be obtained without scanning of the wafer stage. Particularly in the case of the projection exposure apparatus using a light source for generating pulsed laser light such as excimer light, the resolution of image decreased with an increase in the scanning speed of the wafer stage from restriction of pulse frequency, and thus, the scanning speed of wafer stage could not be increased over a specific speed, thus resulting in having a particular disadvantage that the measuring time was long.